1. Field of the Invention
The present invention relates to a method of determining an optimal capacitance of a capacitor used in an air bag system utilizing a bus line, and an air bag system utilizing a bus line.
2. Description of Related Art
An air bag system for protecting a passenger from an impact at a time of collision of a vehicle is indispensable, and air bag system needs to be reduced in weight from a demand of reducing weight of a whole vehicle. Recently, kinds and the total number of air bags such as an air bag for a driver side, an air bag for a passenger side next to the driver, an air bag for a rear seat, and an air bag for side collision are increasing, and therefore, a lighter air bag system is in greater demand.
In a current air bag system, an electronic control unit (ECU) connected to a power source (a battery in a vehicle) and an impact detecting sensor is individually connected to respective gas generators (a gas generator and an air bag are accommodated in a module case). An aspect of the connection between the ECU and the individual gas generators is shown in FIG. 8.
As shown in FIG. 8, the ECU and an igniter (FIG. 9) of each of the individual gas generators are always connected to each other through two conductors (lead wires), and therefore, twice the number of conductors as compared to the total number of igniters are required. Having many conductors contributes largely to weight increase in the air bag system. In view of constraints at a time of assembling vehicle parts, the ECU and the individual gas generators are connected not only by the conductors but by connecting a plurality of conductors via a plurality of connectors, and thereby, there occurs a serious problem such as a weight increase due to these connectors and a cost increase due to increase of the number of the connectors. Further, increase in weight due to increase in volume of the capacitor assembled in the ECU as a backup power source (in case of disconnection between the power source and the ECU) for activating all of the igniters cannot be ignored.
Furthermore, in the air bag system shown in FIG. 8, lead wires and many connectors are interposed between a heat generating portion of the igniter and the ECU. Considering a resistance value due to these members (usually, about 4Ω), a resistance of the heat generating portion can not be set to not more than 1.5Ω in order to detect a short circuit between the heat generating portion and the ECU on the basis of a voltage difference therebetween.
In view of the above, a trial for reducing the weight of a conductor required to connect between the ECU and each gas generator by using a bus system in the air bag system has been examined. An aspect of the air bag system utilizing this bus system is shown in FIG. 1.
As shown in FIG. 1, an air bag system is constituted by providing bus lines comprising a plurality of loop wires passing through the ECU and connecting each gas generator to the bus line through two conductors (three or more conductors when occasion demands). In the case of such an air bag system as shown in FIG. 1, since only a required gas generator is activated according to a collision of a vehicle, an integrated circuit receiving information transmitted from the ECU and a capacitor supplying current to make the heat generating portion in the igniter generate heat are provided in each gas generator. In the case of using a bus system, the total number of capacitors is increased, but since the capacitors are distributed to the ECU and the respective igniters, the capacitance and weight of the capacitor per one igniter is reduced. Therefore, they are remarkably reduced in weight as compared to the capacitor for backup in the air bag system shown in FIG. 8. Accordingly, a large weight reduction is achieved in the whole system in addition to reducing large amount of using conductors, which is expected to be put in a practical use in the air bag system. In this case, as the prior art using the bus system, JP-A 2000-241098, JP-A 2000-513799 and JP-B 2707250 are known.
Further, in the air bag system shown in FIG. 1, since a distance between the heat generating portion of the igniter and the integrated circuit is shorter than a distance between the heat generating portion of the igniter and the ECU shown in FIG. 8 and it is unnecessary to detect a short circuit, the resistance of the heat generating portion of the igniter can be not more than 1Ω. At this time, an amount of power consumed in the heat generating portion may be low, so that a size of the capacitor can be made small. This is much advantageous in view of providing a capacitor or the like in a limited small space inside the igniter. In the air bag system shown in FIG. 1, however, as the resistance value of the heat generating portion of the igniter has been made small, influence of resistance values of other elements cannot be ignored. For this reason, in the air bag system shown in FIG. 1, considering the resistance value of the heat generating portion and the other elements totally and energy to be consumed, it becomes important to optimize the capacitance of the capacitor in order to supply current, which is sufficient for a normal operation, to the heat generating portion.
In JP-A 2001-525288, as an ignition circuit utilizing a bus system and a method of activating the ignition circuit, there is disclosed that a charging voltage to a capacitor is set to be a charging voltage corresponding to 2 to 2.4 times a product of the minimum ignition current and a resistance value of a heat generating portion [V=(2.0-2.4)×IR] and that a capacitor capacitance is set to be 1.0 to 1.2 times the minimum time T divided by an internal resistance R[C=(1.0-1.2)×T/R]. In this related art, however, a resistance value of elements other than the heat generating portion is not accounted and the ground of such a numerical value as 2-2.4 or 1.0-1.2 is not shown, either. Furthermore, although such an extra charging voltage or charging capacitance is expected, the resistance value of elements other than the heat generating portion or the like is not accounted, so that, when an amount of power consumed at these portions is large, the heat generating portion does not generate heat sufficiently.